300-420 Question #332: Real Exam Question with Answer & Explanation

Question

Refer to the exhibit. A network engineer working for a private service provider with an employee ID: 1234:56:789 must design a BGP solution based on: - All traffic originating from AS100 must pass through AS200 to reach the NTP and DHCP server - When a link failure occurs between R3 and R4, traffic must follow the R2-R9 link to reach the NTP and DHCP server Which solution must the design include?

Options

• ARouters R3 and R10 advertise an IGP metric into BGP during redistribution in both directions
• BRouter R6 influences the paths of R9 and R11 to the DC with a higher AS-PATH value
• CRouters R3 and R10 advertise a lower local preference for outgoing traffic and a higher AS-PATH
• DRouter R3 applies a local preference of 200 for R1, R2, R9, and R11 routers to reach the data

Explanation

To achieve specific traffic flow and failover paths for traffic from AS100 to AS300 via AS200, router R6 influences the paths of R9 and R11 to the DC with a higher AS-PATH value.

Common mistakes.

• A. Advertising an IGP metric into BGP during redistribution can influence BGP path selection, but it's typically less effective for influencing external AS traffic than AS-PATH or Local Preference and doesn't directly address route feedback prevention or specific failover path requirements.
• C. Advertising a lower local preference for outgoing traffic would make R3 and R10 prefer other paths out of their own AS, which doesn't align with the requirement to route traffic from AS100 to AS300 or the failover scenario. A higher AS-PATH would make the path less preferred, but combining it with a lower local preference is confusing for the stated goal.
• D. Applying a local preference of 200 on R3 for reaching R1, R2, R9, and R11 would influence R3's outbound traffic preferences, but it doesn't align with the global requirement for traffic from AS100 to AS300 or the specific failover through R2-R9, as local preference typically influences a router's choice of exit point from its own AS.

Concept tested. BGP path manipulation (AS-PATH prepending) for traffic engineering and failover

Reference. https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/20459-bgp-aspath.html

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